Various methods for detecting and amplifying a nucleic acid contained in a biological sample are known. For example, hybridization methods, in which a nucleic acid is detected by the presence or absence of hybridization of a probe and the target nucleic acid, and methods in which a target nucleic acid is amplified by elongating primers.
A cell contains many impurities, such as pigments and proteins, and these impurities inhibit hybridization of a probe or a primer with a target site or in annealing and reactions such as PCR. Therefore, for detection and amplification of a nucleic acid contained in a biological sample, a nucleic acid must be extracted and purified from a biological sample before employing these methods. For extraction and purification of nucleic acid, cells are lysed, and proteins are further removed using phenol, chloroform, or isoamyl alcohol. Alternatively, a method is employed in which proteins are inactivated with a surfactant or a high salt concentration and are degraded with a protease.
For concentration of a nucleic acid or removal of salts, phenol, and the like, ethanol or isopropanol is added to precipitate the nucleic acid. After a nucleic acid is purified by several such procedures including centrifugation steps, hybridization or amplification is performed.
It is known that detection and amplification of nucleic acid extracted from a biological sample such as blood, cells, and feces, and that has been purified, are inhibited by, in particular, impurities contained in the biological sample. It has been confirmed that this inhibitory effect is attenuated by adding bovine serum albumin, single-stranded DNA binding T4 gene 32 protein (gp32), betaine, proteinase inhibitors, and the like (Non-Patent Document 1).
Furthermore, it is known that, as a convenient method for preparing a template for a PCR method, cells are lysed by heating in an alkali and then neutralized to use as a template, or a nucleic acid sample can be obtained by using cells or a tissue mixed with water and heated as a template without being subjected to inhibition in a PCR analysis (Non-Patent Document 1, etc.).
It is known that it is particularly difficult to amplify a nucleic acid having a high GC content by a PCR method and that PCR amplification efficiency can be increased by adding a polyhydric alcohol and/or ammonium sulfate to the PCR reaction mixture in PCR using purified DNA having a high GC content (Patent Document 1).
Furthermore, it is known that, to increase efficiency of the nucleic acid amplification reaction, a biological sample containing the nucleic acid is treated with an acid in any step of nucleic acid extraction to remove substances interfering with nucleic acid amplification, and the obtained nucleic acid extract is used (Patent Document 2).    Patent Document 1: Japanese Patent Laid-Open No. 2001-352982    Patent Document 2: Japanese Patent Laid-Open No. 2003-159056    Non-Patent Document 1: J. Clin. Microbiol. 2000 December; 38(12) 4463-70    Non-Patent Document 2: Bio Techniques, 1998, (4), 580-582